In an extreme ultraviolet (EUV) light source, a droplet generator is used to deliver 10-50 μm droplets of target material, e.g., molten tin, to the focus of the EUV light collecting optics where the droplets are irradiated with laser pulses, thus creating a plasma that produces EUV light. The droplet generator includes a reservoir that holds the molten tin, a nozzle with a micron-sized orifice, and an actuator to drive droplet formation. High purity tin (e.g., 99.999-99.99999% pure) must be used in the droplet generator as even a ppm-level of contamination with certain impurities can lead to the formation of solid particles of a tin compound that are capable of clogging the nozzle and thereby causing the EUV light source to fail.
The purification processes typically used by suppliers for production of tin are generally quite effective for removing impurities formed by chemical elements, e.g., metallic impurities. Such purification processes, however, are not specifically formulated to remove oxygen from tin as oxygen is typically acceptable in most applications of high purity metals. Commercially pure tin contains oxygen at a concentration that significantly (at least about 1,000 times) exceeds the solubility limit of oxygen just above the melting point of tin. Consequently, tin oxide particles are readily formed and, in some instances, cause blocking of the nozzle orifice and in turn failure of the droplet generator and the EUV light source.
It is in this context that embodiments arise.